a) Field of the Invention
This invention relates to equipment for sound reinforcement and reproduction, more specifically, to a loudspeaker.
b) Description of the Prior Art
There has long been a need to reproduce or reinforce speech and music for a variety of applications in public address, instruction and entertainment. This need includes a requirement to reproduce or reinforce programme material having a wide frequency range, and to do so with high fidelity and at substantial volumes. Further, such reproduction or reinforcement must be performed in a wide range of venues, of different sizes and having different acoustic properties, imposing different limitations on the size, number, and possible locations of loudspeaker enclosures. There is generally also the requirement, imposed by such limitations and by economic and other factors, that the desired level of performance be achieved while minimising the total size of the sound system.
In addressing this need, hundreds of different enclosure designs have been developed and produced over more than half a century. As the physics of those transducers in common use make it impractical to reproduce the full range of frequencies both at high power and with high efficiency, virtually all systems divide the full frequency range into two or more bands and employ a different transducer for each. xe2x80x9cCompression driversxe2x80x9d are used for reproducing higher frequencies and xe2x80x9ccone loudspeakersxe2x80x9d for lower frequencies (often of different diameters for different bands). In some cases, each transducer design is packaged in its own enclosure, and the enclosures for the various designs stacked or otherwise assembled at the venue to form a large, full-range, array. In other cases, transducers for several of the frequency bands are integrated in a common enclosure.
The performance of a sound system is a product of not only the transducer designs selected; the particulars of their construction; and the frequency ranges at which they are operated, but by the design and construction of their enclosures. This application is concerned with those enclosures that employ cone loudspeakers, whether discrete or integrated in a common enclosure. While there have been many variations in the design of such enclosures, certain basic approaches account for the vast majority of those in use. Once such approach to enclosure design is the xe2x80x9cdirect radiatorxe2x80x9d (also employed for the cone loudspeaker in virtually all xe2x80x9cbookshelfxe2x80x9d HI-FI speakers); in which the cone loudspeaker is mounted to an opening in one planar side of an enclosure, such that the cone radiates directly into free air. Such enclosures radiate across wide vertical and horizontal angles, and as such, are of value in comparatively small venues in which wide dispersion is desired. They are of limited value in larger venues and outdoors because of the limited efficiency with which they convert a given amount of electrical energy to acoustic energy; lack of projection over distance and, worst of all, because of the amount of mutually destructive interference between the comparatively large number of enclosures required to produce a given sound pressure level.
Another approach to enclosure design is xe2x80x9chorn loadingxe2x80x9d. The cone loudspeaker operates into one end of a channel having acoustically-closed sides and progressively increasing cross-sectional area towards an acoustically-open front end.
One advantage of such an approach can be an improvement in the efficiency with which electrical energy is converted by a given loudspeaker into acoustical energy (relative to the same loudspeaker in a direct radiator enclosure).
The second is a tighter dispersion or higher xe2x80x9cQxe2x80x9d that allows directing the acoustic output towards the listener. These advantages permit horn-loaded loudspeakers to achieve a desired sound pressure level at the listener in larger venues using a smaller number of loudspeakers with less mutual interference. This is offset to some extent by undesirable colouration that has been responsible for the continued use of direct radiator designs in larger venues despite the potential practical advantages of a horn-loaded system. Further, as a result of variations in dispersion for a given horn design over a range of frequencies, horn-loading reduces the range of frequencies a given loudspeaker can reproduce relative to the same loudspeaker in a direct radiator enclosure.
For venues of moderate size and for those portions of large venues (or outdoors) relatively close to the speaker array, there is a need for loudspeaker enclosure of moderately directional character that does not suffer from the known disadvantages of horn-loaded enclosures.
A loudspeaker enclosure is disclosed in which a member is mounted within an acoustic channel having acoustically-closed sides and an acoustically-open front so as to restrict the free-space within the channel. A loudspeaker is arranged to radiate into the channel. The longitudinally-extending member is mounted directly in front of, and in alignment with, the loudspeaker.
Preferably the rear of the longitudinally-extending member effectively extends to at least the surface of the dust excluder within the voice coil diameter, such that the volume defined between the rear of the longitudinally-extending member and the loudspeaker is an annulus and, preferably, that this extension fixes the distance between the two surfaces.
The profile of the longitudinally-extending member is non-circularly symmetric; the width of the member decreasing along its length in one plane, and remaining substantially similar in the other, decreasing rapidly in width at the front. The channel is similar in this respect, and, in combination with the member, presents a lower acoustic impedance in one plane than in the other. The surface of the channel include regions in which the rate at which the surface diverges from the central axis abruptly increases. The point at which this increase occurs changes with position within the channel.
The disclosed enclosure provides the desired degree of projection and dispersion, controlled by the shapes of the member and channel in combination. The efficiency of the enclosure is at least as good as horn-loading but the undesirable colouration is avoided and phase coherance is markedly improved. Both the frequency range over which and the highest frequency at which a given loudspeaker can be used are improved, from which flow other advantages.